Various materials are presented to be used as a solid electrolyte in a solid oxide fuel cell comprising: a solid electrolyte layer that is formed in a layer; a fuel electrode that is formed on a first side of the solid electrolyte layer wherein a fuel gas flows through the fuel electrode to reach the solid electrolyte layer; and an air electrode that is formed on a second side of the solid electrolyte layer wherein an oxidant gas flows through the air electrode to reach the solid electrolyte layer.
In such background, recently, a perovskite-type composite oxide (hereinafter also referred to as “lanthanum gallate oxide”) comprising at least La and Ga has been focused on as a substance that obtains higher oxygen ion conductivity than that of a stabilized zirconia, and a lot of research is being conducted. Lanthanum gallate oxide is a substance in which the decrease in oxygen ion conductivity is small, even at low temperature, and in which part of the La or Ga is replaced with Sr, Mg or the like, which has a lower valence than that of La or Ga, through a substitutional solid solution, resulting in that a sintered body has a property of a large oxygen ion conductivity. As described above, the lanthanum-gallate-based electrolyte using the lanthanum gallate oxide as the solid electrolyte is presented as an example in patent document 1 mentioned below.